1. Field of Invention
The present invention relates generally to a chemical mechanical polishing system. More specifically, the invention relates a method and apparatus for positioning a workpiece or semiconductor wafer into a chemical mechanical polishing system.
2. Background of Prior Art
In semiconductor wafer processing, the use of chemical mechanical polishing, or CMP, has gained favor due to the enhanced ability to stack multiple feature layers on a semiconductor workpiece, or wafer. As the demand for polishing wafers as part of a semiconductor fabrication process increases the requirement for higher processing rates while minimizing risk of wafer damage and contamination has correspondingly experienced greater urgency for improvement.
Two such CMP systems that address these issues are described in a commonly assigned patent to Perlov et al. (U.S. Pat. No. 5,804,507, issued Sep. 8, 1998) and in a commonly assigned patent to Tolles et al. (U.S. Pat. No. 5,738,574, issued Apr. 15, 1998). Both Perlov et al. and Tolles et al. are hereby incorporated by reference. Both Perlov et al. and Tolles et al. disclose a CMP system having a polishing apparatus that is supplied wafers from cassettes located in an adjacent liquid filled bath. A transfer mechanism, or robot, facilitates the transfer of the wafers from the bath to a transfer station. A pedestal within the transfer station rises to receive the wafer from the robot. The wafer is released from the robot and secured, or chucked, to the pedestal by a vacuum chuck. The pedestal is then retracted into the transfer station wherein the wafer is temporarily released from the pedestal so that a plurality of alignment jaws may manipulate the wafer into a position centered upon the pedestal. The wafer is then re-chucked to the pedestal and the pedestal rises to engage and transfer the wafer to a retaining ring of a polishing head. The alignment of the wafer to the center of the pedestal ensures that the wafer will be properly fixtured into the retaining ring. After the wafer is fixtured into the retaining ring, a carousel moves the polishing head to a polishing station. After completion of the polishing process, the wafer is returned to the pedestal where a robot retrieves the wafer and moves it to the proper cassette located in the bath.
Although this process has proven to be an effective mechanism for performing chemical mechanical polishing, an improvement has been identified which could enhance the transfer of the wafer into the polishing head, reducing the risk of wafer damage and contamination, while reducing the dwell time of the wafer at the transfer station.
More specifically, the process of transferring the wafer into and out of a single load/unload pedestal with a transfer robot that can only transport one wafer at a time causes a bottleneck in wafer movement into and out of the polisher. Specifically, a single arm robot with a single end effector for gripping a wafer obtains a wafer from a wafer storage device, positions the wafer upon a single load/unload pedestal. The wafer enters the polisher and work is polished. The wafer is then returned to the load/unload pedestal for unloading and transporting to a wafer storage device. The single arm robot must then grip the processed wafer and transport it to a wafer storage device, place the wafer in the device, and index down and retrieve the next wafer to be processed in the polisher. The robot then transports the next wafer to the load/unload pedestal for drop off. Elimination of the dwell time or the dead time in which the load/unload pedestal is idle would improve the routing time required to process each wafer, and yield a corresponding increase in wafer throughput. Additionally, the use of wafer edge grip only would minimize physical contact with either the backside surface or the device surface of the wafer during transporting and wafer placement. This is of special concern since any abrasion, scratching or other damage could occur on either side of the wafer.
Therefore, there is a need in the art for an apparatus that facilitates wafer transfer into a CMP polishing head wherein the transfer time is reduced and the wafer is fixtured with a minimal risk of particulate contamination and wafer damage.